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    <title>SRFI 11: Syntax for receiving multiple values</title>
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<H1>Title</H1>

SRFI 11: Syntax for receiving multiple values

<H1>Author</H1>

Lars T Hansen

<H1>Status</H1>
This SRFI is currently in ``final'' status.  To see an explanation of each status 
that a SRFI can hold, see <A HREF="http://srfi.schemers.org/srfi-process.html">here</A>.
You can access the discussion via <A HREF="http://srfi.schemers.org/srfi-11/mail-archive/maillist.html">the archive of 
the mailing list</A>.
<P><UL>
<LI>Received: 1999/09/10
<LI>Draft: 1999/09/14-1999/11/12
<LI>Revised: 1999/11/01
<LI>Revised: 2000/03/15
<LI>Final: 2000/03/15
</UL>

<H1>Abstract</H1>

The SRFI introduces syntactic forms LET-VALUES and LET*-VALUES that bind the values of
expressions that return multiple values.

<H1>Issues</H1>

None.

<H1>Rationale</H1>

LET-VALUES and LET*-VALUES reduce the clutter of the CALL-WITH-VALUES notation for
receiving multiple values.


<H1>Specification</H1>

<dl>
<dt><a name="let-values"></a>
<tt>(LET-VALUES ((&lt;formals&gt; &lt;expression&gt;) ...) &lt;body&gt;)</tt>
<br>Syntax
<dd>
<p>Each &lt;formals&gt; should be a formal arguments list as for a LAMBDA 
   expression, cf section 4.1.4 of the R5RS.</p>

<p>The &lt;expression&gt;s are evaluated in the current environment, the 
   variables of the &lt;formals&gt; are bound to fresh locations, the return 
   values of the &lt;expression&gt;s are stored in the variables, the &lt;body&gt; 
   is evaluated in the extended environment, and the values of the
   last expression of &lt;body&gt; are returned.  The &lt;body&gt; is a 
   &lt;tail body&gt;, cf section 3.5 of the R5RS.</p>

<p>The matching of each &lt;formals&gt; to values is as for the matching of 
   &lt;formals&gt; to arguments in a LAMBDA expression, and it is an
   error for an &lt;expression&gt; to return a number of values that does
   not match its corresponding &lt;formals&gt;.</p>

<p>
<pre>
             (let-values (((a b . c) (values 1 2 3 4)))
               (list a b c))            --&gt; (1 2 (3 4))

             (let ((a 'a) (b 'b) (x 'x) (y 'y))
               (let-values (((a b) (values x y))
                            ((x y) (values a b)))
                 (list a b x y)))       --&gt; (x y a b)
</pre>
</p>

<dt><a name="let*-values"></a>
<tt>(LET*-VALUES ((&lt;formals&gt; &lt;expression&gt;) ...) &lt;body&gt;)</tt>
<br>Syntax
<dd>
<p>Each &lt;formals&gt; should be a formal arguments list as for a LAMBDA 
   expression, cf section 4.1.4 of the R5RS.</p>

<p>LET*-VALUES is similar to LET-VALUES, but the bindings are performed
   sequentially from left to right, and the region of a binding indicated
   by (&lt;formals&gt; &lt;expression&gt;) is that part of the LET*-VALUES
   expression to the right of the binding.  Thus the second binding is done in
   an environment in which the first binding is visible, and so on.</p>

<p>
<pre>
             (let ((a 'a) (b 'b) (x 'x) (y 'y))
               (let*-values (((a b) (values x y))
                             ((x y) (values a b)))
                 (list a b x y)))       --&gt; (x y x y)
</pre>
</p>
</dl>

<H1>Implementation</H1>

The following implementation is written in R5RS Scheme.  It is not
compatible with the IEEE Scheme standard because the IEEE standard does
not contain the high-level macro system.

<p>The implementation assumes that some top-level names defined by the
R5RS are bound to their original values.

<p>
<pre>
;; This code is in the public domain.

(define-syntax let-values
  (syntax-rules ()
    ((let-values (?binding ...) ?body0 ?body1 ...)
     (let-values "bind" (?binding ...) () (begin ?body0 ?body1 ...)))
    
    ((let-values "bind" () ?tmps ?body)
     (let ?tmps ?body))
    
    ((let-values "bind" ((?b0 ?e0) ?binding ...) ?tmps ?body)
     (let-values "mktmp" ?b0 ?e0 () (?binding ...) ?tmps ?body))
    
    ((let-values "mktmp" () ?e0 ?args ?bindings ?tmps ?body)
     (call-with-values 
       (lambda () ?e0)
       (lambda ?args
         (let-values "bind" ?bindings ?tmps ?body))))
    
    ((let-values "mktmp" (?a . ?b) ?e0 (?arg ...) ?bindings (?tmp ...) ?body)
     (let-values "mktmp" ?b ?e0 (?arg ... x) ?bindings (?tmp ... (?a x)) ?body))
    
    ((let-values "mktmp" ?a ?e0 (?arg ...) ?bindings (?tmp ...) ?body)
     (call-with-values
       (lambda () ?e0)
       (lambda (?arg ... . x)
         (let-values "bind" ?bindings (?tmp ... (?a x)) ?body))))))

(define-syntax let*-values
  (syntax-rules ()
    ((let*-values () ?body0 ?body1 ...)
     (begin ?body0 ?body1 ...))

    ((let*-values (?binding0 ?binding1 ...) ?body0 ?body1 ...)
     (let-values (?binding0)
       (let*-values (?binding1 ...) ?body0 ?body1 ...)))))
</pre>

<H1>Copyright</H1>
<p>Copyright (C) Lars T Hansen (1999). All Rights Reserved.</p>

<p>
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
</p>
<p>
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
</p>
<p>
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
</p>

    <hr>
    <address>Editor: <a href="mailto:srfi-editors@srfi.schemers.org">Mike Sperber</a></address>
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Last modified: Tue Sep 28 10:59:57 MST 2004
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